Diesel engines typically have higher efficiency than gasoline engines due to an increased compression ratio and a higher energy density of diesel fuel. A diesel combustion cycle produces particulates that are typically filtered from diesel exhaust gas by a diesel particulate filter (DPF) that is disposed in the exhaust stream. Over time, the DPF becomes full and the trapped diesel particulates must be removed. During regeneration, the diesel particulates are burned within the DPF.
One regeneration approach injects fuel into a cylinder after combustion. Post-combustion injected fuel exits the engine with the exhaust gas and is combusted by diesel oxidation catalysts disposed in the exhaust stream. The heat released by the combustion in the catalysts increases the exhaust temperature, which burns the particulates in the DPF. This approach utilizes the common rail fuel injection system and does not require additional fuel injection hardware. However, if not properly controlled, this approach can cause visible white smoke and/or objectionable odor, which is known as hydrocarbon (HC) break-through.
To prevent HC break-through, the amount of fuel delivered during post injection can be controlled based on engine speed and engine load. This approach, however, fails to account for transient conditions. The control method must also be recalibrated any time the relationship of airflow and temperature to engine speed and engine load changes.
In another approach, post injection release timers are used to increase the exhaust temperature. The post injection release timers enable post injection after a pre-set time limit. However, if post injection temperature is not at an ample level and post injection occurs, HC break-through can occur. The amount of time required to heat an exhaust system is dependent upon driving conditions, ambient air temperature, and the age of the exhaust system components. As a result, the pre-set time limit approach does not account for variations in driving conditions.